This application is related to Japanese application No. 2000-334752 filed on Nov. 1, 2000, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a solar cell having pin structure made of compound semiconductor materials and a process of manufacturing the same.
2. Description of Related Art
Most of recent solar cells utilize Si as a starting material. In view of coordination with a solar beam spectrum, solar cells utilizing GaAs, which is a compound semiconductor material, are also applied practically to, for example, a power source of artificial satellites that requires high photoelectric conversion efficiency.
There is a limit to the photoelectric conversion efficiency of a solar cell made of a single material because it only utilizes light of wavelength corresponding to a forbidden band width derived from the material. Under such circumstances, a solar cell of tandem structure made of several materials having different forbidden band widths is developed for obtaining higher photoelectric conversion efficiency. In the tandem solar cell, multiple solar cells are stacked in decreasing order of the forbidden band width from a light receiving face. Accordingly, light of broad wavelength corresponding to the forbidden band widths of the stacked solar cells is utilized.
Another solar cell of multiple quantum well structure is proposed (Journal of Applied Physics vol. 67 p3490 (1990)).
The solar cell comprises an i-type semiconductor layer 103 inserted between a pn junction of an n-type semiconductor layer 2 and a p-type semiconductor layer 4 as shown in FIG. 14. The i-type semiconductor layer 103 includes a barrier layer 130 formed of a semiconductor material for forming the pn junction and a well layer 131 formed of a semiconductor material having a forbidden band width smaller than that of said semiconductor material.
An energy band model of the above-mentioned solar cell of multiple quantum well structure is shown in FIG. 15.
Referring to FIG. 15, Ec and Ev show a lower end of a conduction band and an upper end of a valence band, respectively. With such a structure, not only light corresponding to the forbidden band width of the semiconductor material forming the pn junction but also light corresponding to the forbidden band width of the semiconductor material forming the well layer 131 is utilized for the photoelectric conversion, without reducing an open voltage. Therefore, solar light of longer waveforms contributes to the photoelectric conversion, which allows obtaining a solar cell with improved spectral response characteristics and high output.
Further, Japanese Unexamined Patent Publication Hei 7 (1995)-231108 discloses a solar cell wherein the i-type semiconductor layer in the pin structure is formed such that the forbidden band width thereof is varied stepwise from the p-type region to the n-type region. According to the publication, semiconductor materials of different compound crystal ratios are grown by crystallization by MBE in sequence to produce the forbidden band width varied stepwise.
According to the above-mentioned solar cell of tandem structure, solar cells and tunnel junctions connecting the cells are formed under optimum conditions, which requires an extremely elaborate and complicated manufacturing process. However, a solar cell with high photoelectric conversion efficiency which compensates such a process has not been provided yet.
In the above-mentioned solar cell of multiple quantum well structure, on the other hand, only a light of defined wavelength contributes to carrier excitation because of a quantum level of electrons defined by a thickness of the well layer 131, or the forbidden band width of the material of the well layer 131.
Accordingly, in order to enlarge the wavelength range, a measure of varying the thickness of the well layer stepwise or a measure of varying the ratio of compound crystals in the semiconductor material for forming the well layer must be employed.
For the manufacture of the well layer having a desired thickness and composition ratio by such measures, however, a highly accurate process is also required as the above-mentioned solar cell of tandem structure.
In view of the above-described problems, the present invention has been achieved to provide a solar cell of high photoelectric conversion efficiency and a simplified process of manufacturing the same.
According to the present invention, provided is a solar cell having a p-type semiconductor layer and an n-type semiconductor layer made of a first compound semiconductor material, wherein one or more quantum well layer which is made of a second compound semiconductor material and has a plurality of projections on its surface is formed between the p-type semiconductor layer and the n-type semiconductor layer, the projections being different in size on a single quantum well layer or on any one of the quantum well layers.
According to another aspect of the present invention, provided is a process of manufacturing a solar cell which comprises a p-type and n-type semiconductor layers of a first compound semiconductor material. The process comprises the steps of: forming a p-type or n-type semiconductor layer on a substrate; forming one or more quantum well layer with a second compound semiconductor material; and forming an n-type or p-type semiconductor layer of the first compound semiconductor material; wherein the quantum well layer is formed by providing a base portion and a plurality of projections on the base portion in sequence, and the projections are formed to have different sizes on a single layer or on any one of the quantum well layers.
That is, a feature of the present invention is to insert the quantum well layer which is made of the second compound semiconductor material and has the projections (hereinafter referred to as a quantum dot layer) in a pn junction region of a solar cell of pn structure or in an i-type semiconductor layer of a solar cell of pin structure.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.